Foil filament strain gage



July 11, 1961 Filed Nov. 29, 1957 L. J. WEYMOUTH ETAL FOIL FILAMENT STRAIN GAGE 4 39 ii i i I I/YI/ENTORS 0-IWVM00TH ARTHUR CIRUGE TTOR E Y United States Patent Ofifice 2,992,400 FOIL FILAMENT STRAIN GAGE Leon J. Weymouth, Bedford, and Arthur C. Ruge, Lexington, Mass, assignors to Baldwin-Lima-Hamilton Corporation, a corporation of Pennsylvania Filed Nov. 29, 1957, Ser. No. 699,831 Claims. (Cl. 338-2) This invention relates generally to a bonded filament type strain gage and more particularly to an improved foil type filament gage, of a free-handling type that is especially useful for high temperature applications, and to an improved method of making the same.

In the well-known bonded filament type gage, the filaments are made of either a fine Wire or foil which are suitably bonded throughout their effective length to a surface of a specimen under test. In one type of such gage the filament is mounted upon a suitable backing material that is cemented to a specimen, but these gages are not particularly suitable for high temperatures so that open wire type gages and strippable foil gages have been used by being bonded to a specimen with suitable high temperature cements, e.g., ceramics. High temperatures are considered to be above 400 F. and up to 1800 F. or higher depending upon the availability of suitable cements. The open wire filament gage is what might be termed a free-handling gage because it can be freely handled before application while still retaining its gage configuration and can be placed down into a layer of cement on the specimen and covered over with a top layer of cement all without the need of any preliminary stripping of a supporting membrane as is required for the strippable foil gage. The open wire filament gage has the desirable characteristic of being relatively easily handled but the strippable foil gage has certain superior advantages although extremely difiicult to handle because of the inherent fiimsiness of the foil itself, it being understood that the foil is extremely thin, e.g., less than two or three ten thousandths of an inch thick.

It is an object of our invention to provide an open filament foil gage that has the free handling qualities of the open wire gage and yet avoids the inherent ditficulties incident to the foil filaments becoming distorted or snarled because of their fiimsiness.

A further object is to provide an improved method of manufacturing such a foil filament free-handling type of gage.

Other objects and advantages will be more apparent to those skilled in the art from the following description of the accompanying drawings in which:

FIG. 1 is a plan view of the preferred form of our invention;

FIG. 2 is a sectional view taken substantially on the line 2-2 of FIG. 1, the cross-sectional thickness being greatly magnified for purposes of illustration;

FIG. 3 is an edge elevation showing the gage with a preliminary backing material which is used in one step of the process of manufacture, the thickness of the material being greatly magnified for purposes of illustration;

FIG. 4 is a plan view of a modified arrangement for supporting the foil filaments in a free-handling condition; and

FIG. 5 is a fragmentary sectional view taken substantially on the line 5-5 of FIG. 4, and being similarly magnified as to its thickness.

In the particular embodiments of the invention disclosed herein we have provided in the preferred form of FIG. 1 a gage made from, and wholly contained and supported within, a single sheet of foil 1 of suitable electrical strain responsive material well-known in the art. The gage may be of any suitable configuration but the principles of our free-handling gage are best illus- 2,992,400 Patented July 11, 1961 2 trated in a grid type gage in which the portions 2 to 9 of the strain sensitive filament extend in successively reverse directions. One end of adjacent filaments such as 2 and 3 are connected by a cross member 10 whose Width is preferably substantially the same as the overall width of filaments 2 and 3 or, in other words, the outer edges of section 10 are simply a continuation of the outside edges of the filaments 2 and 3 thereby insuring a high degree of resistance against twisting and snarling of the filament portions. Similar sections 10a are provided at the other end of filament 3 to connect with filament 4, and so on through the remainder of the grid. In case it is desired to have means for adjusting the resistance of the gage grid there are provided first and last filament portions 11 and 12. The disclosure of one of these filament portions will suffice for the Other portion and similar reference numerals will be used for each. The filament portion 11 is connected by narrow sections 15 to a relatively wide lead section 16 which if desired is tapered as at 17 from a wide portion 18. The portions 16 and 18 are in fact all part of a substantially non-strain responsive lead to which an external lead 19 of any suitable material may be soldered, Welded, or otherwise suitably electrically secured to the lead portion 18. The filaments 11 and '12 have their inner edges extend continuously in one direction to form Wide sections 20 generally similar to the sections 10 while the other ends of such filaments are supported by being continued for their full width into the area 21. The connecting portions 15 are formed by the provision of holes 22 in the foil. To adjust the resistance of the gage grid one or more of the connecting portions 15 may be cut or broken. This particular feature does not constitute a part of our present invention except as it is embodied in a particular manner in our improved free-handling gage.

It will be noted that the sections 10, 11, 2t and 21 are continued for their full width to an outer surround-ing frame or perimetric portion 24 completely enclosing the grid and formed out of the same sheet of foil material thereby providing a co-planer support that in the specific embodiment of FIG. 1 is lateral of the end edges of the gage grid. Such sections by constituting substantially a continuation of the outside parallel edges of adjacent filaments at one end thereof perform the function of limiting the twisting of the filament portions 2 to 9 about their lengthwise axis with respect to the frame 24. By this torsional resisting arrangement it has been found that an open filament foil type gage may be freely handled without any supporting backing and with minimum possibility of damaging, snarling or otherwise disarranging the filament. Hence it is possible to easily, quickly, and effectively place this gage on a layer of cement which is initially placed on the surface of a specimen and to have complete assurance that the filaments will lie in their intended configuration which in the specific embodiment is one of parallelism of the filament portions.

After the gage is positioned in the cement it is then only necessary to remove the supporting frame 24. To do this easily the connections 10, 11 and the lead portions 16 are perforated as at 25 and 26. The remaining portions 27 of these sections are then cut along the dotted lines 28 and 29 thereby completely freeing the frame 24 which can then be removed.

The method of making our improved gage consists in spraying or otherwise forming on a single solid sheet of foil 30, FIG. 3, a suitable soluble or otherwise easily removable backing 31. The backing 31 may be of the same thickness as the foil 30, or thicker if desired. To form a gage configuration in the sheet of foil 30 we then employ the well-known photo etching process disclosed in the Eisler Patent No. Re. 24,165 (formerly Patent No. 2,587,568) and other Eisler patents relating to the photo etching process. Hence it is not necessary to describe such process but only to point out that instead of allowing the backing 31 to remain as part of the foil 30 after the gage configuration is'etched, the filament is then dissolved. The backing material 31 can be of any one of many materials such as vinyls, wax, etc. that are liquids at application and thereupon solidify or harden into a thin supporting, film while the dissolving agent may be toluol for wax, and methyl ethyl ketone or acetone for vinyls.

In the modification shown in .FIG. 4 the, gage configuration 33 is supported by anexternal -Cshaped frame 34 of Celluloid or other suitable material that is sufficiently heavy to support the gage until after it is laid down in the cement on a specimen. The gage is mounted on the frame 34 by first forming the gage configuration either on a soluble backing 31 or on a strippable backing 31. In either case the gage 33 would be suitably secured to the frame 34 and then the strippable or the solu ble backing would be removed. It will be noted that the parallel filament portions or strands 35 are connected at their alternate ends by cross-connections 36 which constitute substantially continuations of the outer sides or edges of adjacent filaments at one of their ends. These cross-connections are secured at their tips 37 to a tongue 38 projecting inwardly from one end of the C-frarne 34 While the other sections 39 are connected as at 40 to the other end of the C-frame. The leads 41 are similarly connected as at 42 to the frame 34. External leads 43 are soldered or otherwise suitably secured to the gage lead sections 41. The gage 33 is now fully supported without any backing material in that the grid is laterally supported at its ends only by the frame. Twisting or other distortions of the gage filaments is limited by the continuations of the adjacent parallel strands in the same manner and with the same desirable results as is present in the preferred form of the invention. After the gage is initially embedded in the cement on the specimen surface it may be removed from its supporting frame by cutting across the sections 36 and 39 and across the lead end portions 41 just above the point of the external leads 43.

From the foregoing disclosure it is seen that we have provided an extremely effective means of providing-a freehandling type of foil filament gage to be readily placed in position on a specimen and to be cairied in stock until placed in use all with minimum danger of injuring or distorting the gage while at the same time maintaining.

all of its desirable sensitive and precision qualities so well known in the art of the bonded filament type gage.

It will, of course, be understood that various changes in details of construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

We claim:

1. A strain gage and lateral support therefor comprising a sheet of electrical strain sensitive foil having openings to form an electrical resistance strain responsive filament extending back and forth between a first and last filament strand to provide substantially parallel grid strands electrically connected in series at their alternate ends, said filament being adapted to be bonded throughout its effective length to a member subject to strain, the gage configuration being enclosed within a completely surrounding perimetric portion of the sheet which has extensions inwardly to the series connected ends of the filament strands, said extensions being an integral part of the sheet, and the perimetric portion of the foil sheet having members extending along side of and being laterally spaced from the first and last filament strands, the perimetric portion being severable from the inward extensions after the gage is so bonded.

2. The combination set forth in claim 1 further characterized in that the gage filament has lead portions disposed adjacent to the first and lastfilament strands and connected thereto at spaced points along the length thereof whereby the resistance of the gage may be varied by severing one or more of such spaced connections.

3. The combinations set forth in claim 1 further characterized in that the gage filament has lead portions disposed adjacent to the first and last filament strands and being wide at one end to which external leads may be attached and tapering toward the other end which is connected to the ends of the adjacent filament strands.

4. The combination set forth in claim 1 further characterized in that the gage filament has lead portions disposed adjacent to the first and last filament strands and connected to'the ends of such strands in series therewith, said lead portions also being connected at their ends to the perimetric portion of thefoil sheet.

5. A strain gage having an electrical resistance strain.

responsive foil filament adapted to be bonded throughout its effective length to a member subject to strain, said filament having parallel portions, one end of one portion being cross-connected to the corresponding end of the adjacent portion whose other end is cross-connected to the corresponding end of the next adjacent portion and.

so on to provide a series grid filament, the first and last portions being connected to leads, and a supporting frame extending crosswise of and connected to the cross-connections of the filament portionsand crosswise of and connected to the leads at one of their corresponding ends and thence along at least one side of the gage in laterally spaced relation to the first and last filament portions and thence crosswiseof and connected to the other corresponding ends of the filament portions, thereby to hold each of the opposite ends of .theparallel filament portions and the leads in their relative positions to provide a torsional resisting force over substantially the full width of the filament portions so as to support the filament, the gage grid being severable from the supporting frame afterthe gage is applied to a specimen surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,498,969 Homan June 24, 1924' 1,502,562 Hausrath- July 22, 1924 2,457,616 Van Dyke Dec. 28, 1948 2,706,697 Eisler Apr. 19, 1955 2,734,013 Myers Feb. 7, 1956 2,899,658 Bean Aug. 11, 1959' 

